Method for producing alkali metal monohydridoborates and monohydridoaluminates

ABSTRACT

The invention relates to a method for producing compounds of general formula (A) by reacting an alkali metal hydride with a compound (B) during which the reaction is carried out in the presence of a catalyst that contains boron, whereby: M represents Li, Na, K, Rb or Cs; E represents B or Al, and; X 1 , X 2 , X 3 , independent of one another, represent a secondary or tertiary alkyl group, which is comprised of 2 to 10 atoms, or represent a phenyl group which itself can be alkyl-substituted or they represent an alkoxy group, and the catalyst, which contains boron, or the conversion product thereof with MH is capable of acting as a hydride transfer agent.

[0001] The invention relates to a process for the preparation of alkalimetal monohydridoboranates and -aluminates of the general formula

[0002] wherein

[0003] M=Li, Na, K, Rb or Cs and

[0004] E=B or Al, and

[0005] X¹, X², X³=in each case independently of one another, is

[0006] a 2 to 10 C atom secondary or tertiary alkyl group or

[0007] a phenyl group, which in its turn can be alkyl-substituted, or

[0008] an alkoxy group.

[0009] Alkali metal monohydridoboranates and -aluminates are to someextent known classes of compounds, the members of which have found usesas reagents in chemical synthesis, e.g. as reducing agents. Thus, forexample, commercially available lithium tri-tert-butoxyaluminium hydrideis employed for the chemoselective reduction of acid chlorides toaldehydes or for the stereoselective reduction of asymmetricallysubstituted or cyclic ketones to alcohols (P. Galatsis,“Lithium-tri-tert-butoxyaluminiumhydride in L. A. Paquette,Encyclopaedia of Reagents for Organic Synthesis, J. Wiley & Sons,Chichester 1995, p. 3168-3172).

[0010] In a similar manner, trialkyl borohydrides also serve asdiversely usable reducing agents in organic synthesis. In general, theirstereoselectivity increases with the steric bulkiness of the alkylsubstituents. (H. C. Brown, S. Krishnamurthy, J. L. Hubbard, J. Am.Chem. Soc. 1978, 100, 3343; R. Köster, “AnionischeOrganobor-Wasserstoff-Verbindungen [Anionic Organoboron-HydrogenCompounds]” in: Houben-Weyl, Methoden der organischen Chemie [Methods ofOrganic Chemistry] 13/3b, p. 798-813, G. Thieme Verlag, Stuttgart, 1983;J. L. Hubbard, “Lithium tri-s-butylborohydride” in: L. A. Paquette,Encyclopaedia of Reagents for Organic Synthesis, J. Wiley & Sons,Chichester 1995, 3172-3176; J. D. Odom, in: Comprehensive OrganometallicChemistry, G. Wilkinson (ed.), Pergamon Press 1982, vol. 1, p. 297).

[0011] Some representatives of the alkali metal monohydridoboranates and-aluminates can be obtained by addition of alkali metal hydride (MH) tothe EX¹X²X³ compound. This applies e.g. to the preparation of Li[HBEt₃]in accordance with

[0012] where Et=ethyl.

[0013] Without a solvent or with a hydrocarbon as the solvent, thereaction at 200° C. takes approx. 4 h, and with Et₂O as the solvent thereaction on heating under reflux takes approx. 24 h (R. Köster,“Anionische Organobor-Wasserstoff-Verbindungen” in: Houben-Weyl,Methoden der organischen Chemie 13/3b, p. 798-813, G. Thieme Verlag,Stuttgart, 1983). In THF solution, the addition can be effected withinone day at room temperature.

[0014] The induction time of the reaction presents problems. This methodalso fails if boranes with bulky substituents are employed. For example,the reaction of B(^(s)Bu)₃ with alkali metal hydrides in boiling THFgives a conversion of only 10% after 24 h, and for this reason thisreaction is unsuitable for a commercial synthesis. (H. C. Brown, S.Krishnamurthy, J. L. Hubbard, J. Am. Chem. Soc. 1978, 100, 3343; R.Köster, “Anionische Organobor-Wasserstoff-Verbindungen” in: Houben-Weyl,Methoden der organischen Chemie 13/3b, p. 798-813, G. Thieme Verlag,Stuttgart, 1983; J. L. Hubbard, “Lithium tri-s-butylborohydride” in: L.A. Paquette, Encyclopaedia of Reagents for Organic Synthesis, J. Wiley &Sons, Chichester 1995, 3172-3176; J. D. Odom, in: ComprehensiveOrganometallic Chemistry, G. Wilkinson (ed.), Pergamon Press 1982, vol.1, p. 297).

[0015] Boranes with even bulkier substituents are inert with respect to“normal”, i.e. commercially obtainable, NaH and LiH in boiling THF. Thisdoes not apply to the highly reactive form of the binary hydrides, suchas are prepared e.g. by decomposition of alkyllithium solutions under ahydrogen atmosphere. (R. Pi, T. Friedl and P. v. R. Schleyer, J. Org.Chem. 1987, 52, 4299-4304). Because the active metal hydride first hasto be prepared from expensive organolithium solutions, this process isof little commercial interest.

[0016] Another variant of preparing active metal hydride comprisespreparing the metal, preferably in finely divided form, in the presenceof a trisubstituted boron compound, so that the hydride MH formed insitu can add on to the boron compound, immediately, to form aborohydride of the formula M[R¹R²R³B]H. Disadvantages in this case arethat the metal must be present in the form of a highly reactive powderwhich is difficult to handle, and a catalyst combination in the form ofa transition metal salt (e.g. FeCl₃) and/or polyaromatics (e.g.phenanthrene) must be employed to achieve reasonable reactiontemperatures and times (U.S. Pat. No. 5,886,229). The product solutionsaccordingly are contaminated and are discoloured by the transition metalcontent.

[0017] Trialkoxy-element hydrides with bulky substituents also do notreact or react only extremely slowly with MH. For example, thepreparation of lithium tri-tert-butoxyaluminium hydride (LTTBA) inaccordance with

[0018] is unknown (see also Comparative Example A).

[0019] Since the direct preparation is not possible, a number of processalternatives have been developed. Thus, LTTBA is prepared by alcoholysisof lithium aluminium hydride in accordance with

[0020] The high preparation costs are a disadvantage, since relativelyexpensive hydride hydrogen in the LiAlH₄ is destroyed by thealcoholysis.

[0021] Trialkyl borohydrides with bulky organic radicals are prepared byone of the following general processes:

[0022] Disadvantageous with the process according to (4) are the use ofexpensive LiAlH(OMe)₃, which is not commercially obtainable, and aboveall the fact that large amounts of insoluble aluminium methylate areobtained, which makes preparation of the trialkyl borohydride in a pureform extremely difficult. Similar circumstances apply to process (5),and in addition there are high costs for the donor, such as e.g.1,4-diazabicyclo[2,2,2]octane (DABCO).

[0023] Process (6) has the disadvantage that expensive t-butyllithium isused as the LiH source, a gaseous by-product being formed. Furthermore,the reaction must be carried out at very low temperatures, which is veryunfavourable in energy terms.

[0024] All the processes (3)-(6) have the disadvantage that they arelimited in practice to the preparation of the lithium derivative, sinceonly the corresponding lithium raw materials (and not the Na or Kcompounds) are commercially obtainable. The addition of higher alkalimetal hydrides (NaH, KH, RbH, CsH) to a starting compound EX¹X²X³ indeedproceeds substantially faster than in the case of LiH, but in thesecases also the rate of reaction decreases sharply with increasing volumeof the substituents X (see comparison example B).

[0025] The object of the invention is to overcome the disadvantages ofthe prior art and to provide a process for the rapid preparation ofalkali metal monohydridoboranates and -aluminates of the general formula

[0026] at mild temperatures which starts from commercially availablealkali metal hydride, allows a reaction procedure without increasedpressure and avoids the formation of insoluble by-products.

[0027] The object is achieved by the process described in claim 1.Claims 2 to 11 develop the process described. Claim 12 describespreferred process products and claim 13 describes a use of the compoundsprepared by the process according to the invention.

[0028] It has been found that the addition described above of alkalimetal hydride (MH) on to an EX¹X²X³ compound is significantlyaccelerated by a catalyst:

[0029] where

[0030] M=Li, Na, K, Rb or Cs and

[0031] E=B or Al and

[0032] X¹, X², X³, in each case independently of one another, =

[0033] a secondary or tertiary alkyl group consisting of 2 to 10 C atomsor

[0034] a phenyl group, which in its turn can be alkyl-substituted, or

[0035] an alkoxy group.

[0036] Any boron-containing compound which contains the structural unitBH₃ and which itself or the reaction product of which with MH is capableof acting as a hydride transfer agent can be employed as the catalyst.

[0037] X¹, X² and X³, in each case independently of one another, canpreferably be iso-propyl or sec-butyl or tert-butyl or tert-amyl orsiamyl (sec-2-methyl-butyl) or a phenyl group, which in its turn can bealkyl-substituted, or the following alkoxy group

[0038] where R^(′1), R^(′2), R^(′3), independently of one another,=alkyl having 1 to 10 C atoms.

[0039] In the case of liquid compounds EX¹X²X³, the reaction can inprinciple be carried out without a solvent; however, working in asolvent is preferred, or unavoidable in cases where EX¹X²X³ is notliquid. Aprotic organic compounds, such as e.g. hydrocarbons and/orethers, are used as the solvent. The reaction proceeds faster in polarsolvents than in non-polar hydrocarbons.

[0040] In principle, the sequence of the addition of the individualreaction partners plays no role. Preferably, the total amount of MH issuspended in the solvent, the catalyst is added and the compound EX¹X²X³is metered in as a function of the rate of reaction.

[0041] Borane complexes H₃B.D with a donor compound D are particularlysuitable as the catalyst. Amines, preferably secondary amines, can beemployed e.g. as the donor compound.

[0042] Aminoborohydrides of the general formula

M[R²R¹NBH₃]_(n)  (D)

[0043] where M=Li, Na, K, Rb, Cs or Mghalogen if n=1, or M=Mg if n=2 and

[0044] R¹, R²=independently of one another H, alkyl or aryl, wherein R¹and R² can be bonded to one another via a ring closure,

[0045] or

M[R²R¹NBH₂NR³R⁴BH₃]_(n)  (E)

[0046] where M=Li, Na, K, Rb, Cs or Mghalogen if n=1 or M=Mg if n=2 and

[0047] R¹, R², R³, R⁴=independently of one another H, alkyl or aryl,wherein R¹, R², R³ and/or R⁴ can be bonded to one another via a ringclosure,

[0048] can also be employed as the catalyst.

[0049] The catalyst is in general employed in an amount of 0.1 to 20 mol%, and in individual cases it may prove favourable to choose an evenhigher dosage. The catalyst is preferably employed in an amount of 0.5to 5 mol %.

[0050] The temperature of the reaction is 0 to 150° C., depending on thesubstrate and solvent.

[0051] The process according to the invention has the advantage thatalkali metal monohydridoboranates and -aluminates can be prepared fromcommercial alkali metal hydrides in a simple manner, rapidly and withoutthe use of pressure. In particular, the alkali metalmonohydridoboranates and -aluminates with sterically voluminoussubstituents are accessible to synthesis in this manner.

[0052] An alkali metal tri-tert-butoxyaluminium hydride or an alkalimetal tri-sec-butyl borohydride or alkali metaltri-(sec-2-methyl-butyl)-borohydride can preferably be obtained ascompound (A) by the process according to the invention.

[0053] The alkali metal monohydridoboranates and -aluminates prepared bythe process according to the invention are used as reducing agents inorganic synthesis.

[0054] The invention is explained in more detail in the following withthe aid of examples.

EXAMPLE I Preparation of Lithium tri-tert-butoxyaluminium HydrideLi[HAl(O^(t)Bu)₃] in THF/Toluene with the Catalyst LithiumDimethylaminoborohydride Li[Me₂NBH₃] (LiDMAB)

[0055] The catalyst solution was first prepared as follows: 7.41 g (1.07mol) lithium metal granules were suspended in 277 g anhydrous THF in a 1l reactor. A mixture comprising 68 g dimethylaminoborane (DMAB), 39.5 gisoprene and 116 g THF was then metered in, while stirring, in thecourse of 2 hours. The reactor internal temperature was adjusted to 22to 27° C. by external cooling.

[0056] After an after-reaction time of two hours at approx. 25° C., thesmall residues of lithium metal were filtered off.

[0057] Yield: 363 g with a Li content of 1.89% (corresponds to 92% oftheoretical)

[0058] δ¹¹B: 14.2 ppm, quartet

[0059] 243 g of a 24.9% Al(O^(t)Bu)₃ solution (242 mmol) in THF/toluenewere initially introduced into a 1 l double-walled reactor with anintensive cooler, KPG stirrer and thermocouple, 2.2 g (278 mmol) LiHpowder and 2.0 g of the solution of LiDMAB in THF prepared above(concentration 2.7 mmol/g, 2.0 g solution correspond to 5.6 mmolLiDMAB=2.3 mol %, based on the educt) were added and the mixture washeated under reflux.

[0060] After two and three hours, respectively, samples were taken andtested for the progress of the reaction by ²⁷Al—NMR. It was found herethat the reaction was already substantially concluded after one hour.After 2 hours the desired product (δ²⁷Al=78.2 ppm) was formed with aspectroscopic purity of ≧90%. The cloudy solution was filtered over a G2glass filter frit (30 min) and analysed.

[0061] Yield: 284 g of clear yellowish solution

[0062] Gas volumetry: H=0.781 mmol/g

222 mmol (

92% of theoretical)

COMPARATIVE EXAMPLE A Preparative Experiment for Li[HAl(O^(t)Bu)₃],without Catalyst

[0063] 35 g of a 30% solution of aluminium tert-butylate (corresponds to43 mmol) in THF/toluene (1:1.6) were initially introduced into a 100 ml2-necked Schlenk flask with a reflux condenser and temperature probe,and 0.70 g (88 mmol) of ground lithium hydride was added. The mixturewas refluxed for three hours (internal temperature 97° C.).

[0064] After cooling to room temperature, a sample was investigated:

[0065] Gas volumetry (filtered sample): not detectable

[0066] δ²⁷Al: 49.1 ppm (of Al(O^(t)Bu)₃)

[0067] no traces of the product peak at 78 ppm

EXAMPLE 2 Preparation of Lithium tri-sec-butyl Borohydride in THF withthe Catalyst LiDMAB

[0068] 7.5 g LiH (0.94 mol) were suspended in 540 g THF in a 1 ldouble-walled reactor and 8.0 g of the catalyst solution prepared inExample 1 (concentration 1.05 mmol/g, 8.0 g solution correspond to 8.4mmol LiDMAB=0.96 mol %, based on the educt) were added. Thereafter,metering in of tri-sec-butylborane (159 g, 875 mmol in total) wasstarted. During this, the reaction temperature rose from approx. 25to >30° C. When the addition had ended, the mixture was boiled underreflux for 1 hour.

[0069] After cooling, the solution was filtered over a glass frit.

[0070] Yield: 690 g of solution with 1.23 mmol/g active hydrogen content(97% of theoretical)

[0071] δ¹¹B: −6.8 ppm, d¹J(B—H)=540 Hz

[0072] Purity (based on the boron species): approx. 97%

EXAMPLES 3 TO 7 Preparation of Lithium tri-sec-butyl Borohydride inVarious Solvents and with Various Catalysts.

[0073] Analogously to Example 2, lithium tri-sec-butyl borohydride wasprepared in various solvents and with various catalysts. Further detailsare to be found in Table 1.

[0074] The catalyst MgDMAB (ClMg[Me₂NBH₃]) was prepared as follows:

[0075] 2.95 g (50 mmol) dimethylaminoborane were dissolved in 8.5 g THFin a 0.1 l flask and the solution was cooled to approx. 10° C. with awater bath. 19.2 g of a 25% ethylmagnesium chloride solution (54 mmol)were metered in with a syringe in the course of 15 minutes. During this,the reaction mixture heated up to 35° C. A gas (ethane) escaped.

[0076] After the mixture had been stirred for one hour at roomtemperature, a sample was taken and investigated by NMR spectroscopy.

[0077] δ¹¹B: 15.5 ppm (quartet)

EXAMPLES 8 AND 9 AND COMPARATIVE EXAMPLE B Preparation of Sodiumtri-sec-butyl Borohydride in Various Solvents and with Various Catalystsand without a Catalyst

[0078] Analogously to Example 2 sodium tri-sec-butyl borohydride wasprepared in various solvents and with various catalysts. Further detailsare to be found in Table 2.

[0079] The catalyst NaDADB (sodium diaminodiborate, Na[Me₂NBH₂NMe₂BH₃])was prepared as follows:

[0080] 46.4 g of a mixture of sodium and aluminium oxide (9.2% Na,corresp. to 171 mmol) were suspended in 150 ml THF in a 250 mltwo-necked flask, and a solution of 10.4 g (176 mmol)dimethylaminoborane (DMAB) in 60 ml THF was added, while stirring, inthe course of 100 min. The reaction mixture heated up to 38° C., withsometimes vigorous evolution of gas (hydrogen). After the mixture hadbeen after-stirred for one hour at room temperature, the Al₂O₃ wasfiltered off.

[0081] 146 g of a solution with an Na content of 0.52 mmol/g wereobtained.

[0082] (Yield: 86%, based on DMAB)

[0083] δ¹¹B: 2.1 ppm (triplet); 14.3 ppm (quartet) intensity ratio 1:1TABLE 1 Preparation of lithium tri-sec-butyl borohydride Startingsubstances Catalyst Reaction B(^(S)Bu)₃ LiH Amount time temp. Yield²⁾Eq. g/mmol mmol Nature¹⁾ mmol/ mol % Solvent min ° C. % 3 9.2/50 64LiDMAB 4.9 10 1,2-dimethoxy- 15 60 28 4 27.6/152 180 LiDMAB 9 6 ethaneEt₂O/THF 180 approx. 42 (5.5:1) 45 5 4.6/25 27 LiMAB 4 16 THF 15 65 51700 25 98 6 9.1/50 64 MgDMAB 5.1 5 THF 120 65 100 7 9.8/54 60 DMAB 5.3 5THF 120 65 100

[0084] TABLE 2 Preparation of sodium tri-sec-butyl borohydride Startingsubstances Catalyst Reaction B(^(S)BU)₃ NaH Amount time temp. Yield²⁾Example g/mmol mmol Nature¹⁾ mmol/ mol % min ° C. % 8 16.9/93 100 LiDMAB7.8 8 10 20-65³⁾ 100 9 14.6/80 100 NaDADB 5.6 7 10 20-65³⁾ 100 B l4.6/80100 ./. ./. ./. 40 65 90 100 65 97

[0085] It can be seen from Examples 3 and 4 (Table 1) that in additionto pure THF or a THF/toluene mixture (as in the preceding examples 1 and2), 1,2-dimethoxyethane and THF/diethyl ether mixtures can also be usedas the solvent.

[0086] Example 5 shows the possible use of a lithium aminoborohydridecatalyst with a cyclic amino radical. In this case the reaction was alsocarried out at two different temperatures: initially at the boilingpoint, only 15 minutes being required for an approx. 50% degree ofconversion. The remainder of the reaction was effected by stirring atroom temperature (over a correspondingly longer period of time).

[0087] Examples 6 and 7 show the use of various other catalysts. Bothchloromagnesium aminoborohydride and dimethylaminoborane lead to aquantitative conversion to the desired product in boiling THF in thecourse of 2 hours.

[0088] Experiments for the preparation of sodium tri-sec-butylborohydride are described in Examples 8 and 9 and Comparative Example B(Table 2). Without a catalyst (Comparative Example B), the mixture mustbe refluxed for about 40 minutes in order to achieve a 90% degree ofconversion. However, the reaction is considerably faster than in theanalogous preparation of the lithium derivative (10% conversion afterstirring under reflux conditions for 24 hours).

[0089] If catalysts such as LiDMAB or a diaminodiboranate (Examples 9and 10) are employed according to the invention to accelerate thereaction, a considerable shortening of the reaction times is to beobserved. The activity of the catalysts also manifests itself in thatthe reaction mixtures heat up to the boiling point directly after theaddition, without external action of heat; this indicates an extremelyfast reaction.

1. Process for the preparation of compounds of the general formula

by reaction of an alkali metal hydride with a compound

characterized in that the reaction is carried out in the presence of aboron-containing catalyst, with M=Li, Na, K, Rb or Cs and E=B or Al andX¹, X², X³=in each case independently of one another being a 2 to 10 Catom secondary or tertiary alkyl group or a phenyl group, which in itsturn can be alkyl-substituted, or an alkoxy group and with theboron-containing catalyst containing the structural unit BH₃ and theboron-containing catalyst or reaction product thereof with MH beingcapable of acting as a hydride transfer agent.
 2. Process according toclaim 1, characterized in that X¹, X², X³=in each case independently ofone another iso-propyl or sec-butyl or tert-butyl or tert-amyl or siamyl(sec-2-methyl-butyl) or a phenyl group, which in its turn can bealkyl-substituted, or the following alkoxy group

where R^(′1), R^(′2), R^(′3)=independently of one another alkyl having 1to 10 C atoms
 3. Process according to claim 1 or 2, characterized inthat the reaction is carried out in an aprotic organic solvent. 4.Process according to claim 3, characterized in that the solvent ispolar.
 5. Process according to one of claims 1 to 4, characterized inthat the boron-containing catalyst is a borane complex of the generalformula H₃B.D  (C) where D=donor compound.
 6. Process according to claim5, characterized in that the donor compound is an amine.
 7. Processaccording to claim 6, characterized in that the donor compound is asecondary amine.
 8. Process according to one of claims 1 to 4,characterized in that the catalyst is an aminoborohydride of the generalformula M[R²R¹NBH₃]_(n)  (D) where M=Li, Na, K, Rb, Cs or Mghalogen ifn=1 or M=Mg if n=2 and R¹, R², independently of one another, =H, alkyl(having 1 to 10 C atoms) or aryl, wherein R¹ and R² can be bonded to oneanother via a ring closure.
 9. Process according to one of claims 1 to4, characterized in that the catalyst is an aminoborohydride of thegeneral formula M[R²R¹NBH₂NR³R⁴BH₃]_(n)  (E) where M=Li, Na, K, Rb, Csor Mghalogen if n=1 or M=Mg if n=2 and R¹, R², R³, R⁴, independently ofone another, =H, alkyl (having 1 to 10 C atoms) or aryl, wherein R¹, R²,R³, R⁴ can be bonded to one another via a ring closure.
 10. Processaccording to one or more of claims 1 to 9, characterized in that thecatalyst is employed in an amount of 0.1 to 20 mol %.
 11. Processaccording to claim 10, characterized in that the catalyst is employed inan amount of 0.5 to 5 mol %.
 12. Process according to one or more ofclaims 1 to 11, characterized in that an alkali metaltri-tert-butoxyaluminium hydride or an alkali metal tri-sec-butylborohydride or alkali metal tri-(sec-2-methyl-butyl) borohydride isobtained as compound (A).
 13. Use of the compounds (A) prepared by theprocess according to claims 1 to 12 as reducing agents.